1. Field of the Invention
The invention relates to an apparatus for tissue stimulation, and in particular a heart pacemaker, for detection of a response to a stimulation pulse when the measured signal is corrupted by an electrode polarisation artefact.
2. Description of the Prior Art
It is desirable that pacemakers have a low energy consumption such that the battery lasts longer. To enable the reduction of the energy consumption it must be clear whether there has been a capture (=a heart contraction/evoked response) or not at the prevailing stimulation voltage. For a proper detection of capture, it is important that the artefact at stimulation, i.e. the polarization voltage, is not so large that it is detected as capture. If the polarization voltage could be eliminated the detection of capture would be easier and more reliable.
U.S. Pat. No. 4,543,956 discloses a system for detecting the evoked response in which the polarization is neutralized using a biphasic waveform technique whereby a compensating current pulse is transmitted in the opposite direction from the stimulating current pulse. However, the compensating current pulse is often emitted very close in time to the stimulation pulse, the recharge pulse thus tending to mask the electrical response from the heart and in particular in the case where a unipolar electrode is used.
U.S. Pat. No. 5,431,693 discloses a pacemaker for detecting capture based on the observation that the non-capture potential is exponential in form and the evoked response potential, while generally exponential in form, has one or more small-amplitude perturbations superimposed on the exponential waveform and whereby the perturbations are enhanced for ease of detection. The perturbations involve relatively abrupt slope changes, which are enhanced by processing the waveform signal differentiation. Abrupt slope changes in the second derivative are used to detect morphological features indicative of capture which are otherwise difficult to discriminate.
U.S. Pat. No. 5,165,405 relates to a pacemaker comprising means for acquiring the curve of the polarisation phenomenon by stimulating the tissue with a stimulation energy lying below the stimulation threshold, so that the electrical potential signal in the tissue subsequently acquired by the detector means corresponds to the polarization phenomena produced by the stimulation attempt without these having an evoked response of the tissue superimposed thereon. By regularly updating the polarisation signal, an optimal compensation of the polarisation components contained in the acquired electrical potential signal is achieved for the purpose of detecting an evoked response.
U.S. Pat. No. 5,417,718 defines a pacemaker that includes a so called Autocapture.TM. system for automatically maintaining the energy of the stimulation pulses generated by the pacemaker at a predetermined level safely above that needed to effectuate capture. The Autocapture.TM. system performs its function by comparing the electrical evoked response of the heart following the generation of a stimulation pulse to a polarization template determined by a capture verification test. During the capture verification test, the Autocapture.TM. system causes the pacemaker to first generate a series of pacing pulse pairs. The first pulse of the pair has a high energy to ensure capture. The second pulse of the pair is of the prescribed stimulation energy. The signal corresponding to the second pulse (which signal is dominated by polarization information) is sensed through a sensing circuit having a specified sensitivity setting. Such signal is stored as the polarization template corresponding to that particular energy and sensitivity setting. In view of the lead polarization signal not being easily characterised, due to it being a complex function of e.g. the lead materials, lead geometry, tissue impedance, stimulation energy, most of which are continuously changing over time, the capture verification test creates a table of polarization templates as a function of sensitivity settings for a particular stimulation energy.